Tech debt: Implement or remove unfinished processor /upload/chunk endpoint #431

Summary: apps/processor/src/api/upload.ts exposes POST /chunk but returns 501 not implemented. Evidence: apps/processor/src/api/upload.ts:415-418 and docs/2.0-architecture/2.6-features/file-upload.md lists chunked upload as future enhancement. Prompt: either implement chunked upload end-to-end (frontend contract + processor persistence + queue integration + error handling) or remove/disable this endpoint and document unsupported behavior. Acceptance criteria: endpoint is production-ready or removed, docs match actual behavior, tests cover chosen path.